Automatic dimming mirror using semiconductor light sensor with integral charge collection

ABSTRACT

Automatically dimming rearview mirrors allow a vehicle operator to view a scene generally behind the operator at a reflectivity level automatically adjusted to ambient light and glare levels. An automatically dimming mirror includes a dimming element having a variably reflective surface, the degree of reflectivity based on a control signal. An ambient light sensor is positioned to receive light from a region generally in front of the vehicle. The ambient light sensor outputs a discrete ambient light signal based on the amount of light incident on the ambient light sensor over an integration period. A glare sensor is positioned to view the scene generally behind the vehicle operator. The glare sensor outputs a discrete glare signal based on the amount of light incident on the glare sensor over an integration period. A dimming logic determines an ambient light level based on the ambient light signal. The glare integration period is determined based on the ambient light level. The glare signal resulting from the glare integration period determines a mirror glare level used to set the dimming element control signal. The use of variable integration times increases the dynamic range of the light sensors, improving the mirror dimming response.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 09/236,969, filed Jan. 25, 1999, titled “AutomaticDimming Mirror Using Semiconductor Light Sensor With Integral ChargeCollection”, the specification of which is expressly incorporated hereinby reference in its entirety.

TECHNICAL FIELD

The present invention relates to automotive rearview mirrors thatautomatically dim in response to glare.

BACKGROUND ART

Vehicle operators use interior and exterior rearview mirrors to viewscenes behind the vehicle without having to face in a rearward directionand to view areas around the vehicle that would otherwise be blocked byvehicle structures. As such, rearview mirrors are an important source ofinformation to the vehicle operator. Bright lights appearing in a scenebehind the vehicle, such as from another vehicle approaching from therear, may create glare in a rearview mirror that can temporarilyvisually impair or dazzle the operator. This problem is generallyworsened during conditions of low ambient light such as occur at night,when the eyes of the vehicle operator have adjusted to the darkness.

Various solutions have evolved to deal with the problem of glare inautomotive rearview mirrors. Initially, a prismatic mirror was providedthat could be manually changed between a setting of high reflectivityand a setting of low reflectivity. When the operator experienced glare,the operator could manually change the rearview mirror setting to lowreflectivity to reduce the glare. The operator could then manuallyswitch the rearview mirror back to high reflectivity after the glare hadsubsided. Difficulties with manually controlled mirrors include theglare experienced before the mirror could be switched as well asoperator distraction caused by finding and operating the mirror control.

Automatically dimming rearview mirrors were designed to eliminate theneed for the operator to manually switch the mirror. The earliestdesigns used a single glare sensor facing rearward to detect the levelof light striking the mirror. This design proved to be inadequate sincethe threshold perceived by the operator for dimming the mirror, known asthe glare threshold, varied as a function of the ambient light level. Animprovement included a second light sensor for detecting the ambientlight level. The glare threshold in these systems is based on the amountof ambient light detected. Among the dual sensor designs proposedinclude those described in U.S. Pat. Nos. 3,601,614 to Platzer;3,746,430 to Brean et al.; 4,580,875 to Bechtel et al.; 4,793,690 toGahan et al.; 4,886,960 to Molyneux et al.; 4,917,477 to Bechtel et al.;5,204,778 to Bechtel; 5,451,822 to Bechtel et al.; and 5,715,093 toSchierbeek et al.; each of which is hereby incorporated by reference.

Another related approach is described in U.S. Pat. No. 5,760,962 toSchofield et al., which is hereby incorporated by reference, in which animaging array is used to gather light from behind and beside thevehicle. Ambient light is detected by examining pixels generally lookingsideways. The cost of the imaging array, the required lens, andcomplicated signal processing logic make the use of an imaging arrayprohibitively expensive for many automotive applications. Also, lightcollected from a side view less accurately represents the ambient lightexperienced by the vehicle operator than does light from a forward view.

Improvements in glare reduction additionally occurred when prismaticmirrors having two states were replaced with mirror assemblies includingdimming elements capable of providing many levels of reflectivityreduction. The most commercially successful type of variable dimmingelement is the electrochromic dimmer. Electrochromic devices producevarious levels of opaqueness in response to an applied voltage. Whenplaced in front of a reflective surface, an electrochromic deviceprovides a variably reflective surface, the degree of reflectivity basedon a control voltage. Typical electrochromic mirrors are described inU.S. Pat. Nos. 4,902,108 to Byker and 5,724,187 to Varaprasad et al. aswell as in commonly assigned U.S. patent applications Ser. No.08/616,967 titled “Improved EC Layer And Devices Comprising Same” toTonar et al., filed Mar. 15, 1996; and U.S. patent application Ser. No.08/832,596 titled “An Improved Electrochromic Medium Capable ofProducing A Pre-Selected Color” to Baumann et al., filed Apr. 2, 1997,each of which is hereby incorporated by reference.

A key element in the design of an automatic dimming mirror is the typeof light transducer used to implement ambient light and glare detection.A primary characteristic of interest in selecting a light transducertype is the dynamic range. The ratio between the intensity of brightsunlight and moonlight is roughly 1,000,000:1, indicating the wide rangethat must be sensed by the ambient light sensor. Both the ambient lightand the glare light sensors must operate within the ranges oftemperature, humidity, shock, and vibration experienced within a vehiclepassenger compartment. If a sensor is to be mounted in an outsidemirror, even harsher operating conditions can be expected. Sensors andsupport electronics must also be inexpensive to allow the cost of anautomatically dimmed mirror to fall within the range deemed acceptableby an automobile purchaser. Transducers should have good noise immunityor be compatible with noise compensation electronics within the sensorfor sensitivity at low light levels. Transducers should further have aspectral response similar to the frequency response of the human eye. Asa final desirable characteristic, the sensor must be easily integratableinto the types of digital control systems commonly found in automotiveapplications.

One type of light transducer is the cadmium sulfide (CdS) cell. CdScells are photosensitive resistors exhibiting increasing conductancewith increasing light levels. CdS cells have the advantages of being lowin cost, having good sensitivity to low light levels, and having aspectral response somewhat similar to that of the human eye.Disadvantages with CdS cells include a high degree of variance betweencells, slow response at low light levels, poor environmental stability,and difficulty being assembled by automated electronic manufacturingequipment. CdS cells for sensing ambient light and glare may beincorporated into a full or partial bridge to increase dynamic range.However, the bridge output represents a fixed relationship betweenambient light and glare that is often not appropriate throughout therange of ambient light levels.

Another type of light transducer used in automatic dimming mirrors isthe discrete photodiode configured as a light-dependent current source.Photodiodes have less variance between parts, better environmentalstability, and are more easily adapted to automated manufacturing thanare CdS cells. However, photodiodes tend to be expensive and producevery low currents at low light levels. These low currents requirespecial amplification techniques to achieve a useful signal, increasingthe cost of the automatic mirror.

A relatively new type of light sensor incorporates a silicon-based lighttransducer and conditioning electronics on a single substrate. The lighttransducer generates charge at a rate proportional to the amount ofincident light. This light-induced charge is collected over anintegration period. The resulting potential indicates the level of lightto which the sensor is exposed over the integration period. Lightsensors with integral charge collection have many advantages. By varyingthe integration time, the sensor dynamic range is greatly extended.Also, the ability to incorporate additional electronics on the samesubstrate as the transducer increases noise immunity and permits thesensor output to be formatted for use by a digital circuit. Componentintegration additionally reduces the system cost. Silicon devices aremore temperature invariant than CdS cells and can be packaged to providethe necessary protection from humidity, shock, and vibration. Onedisadvantage of silicon-based light transducers is a frequency responsedifferent than that of the human eye. Types of charge accumulating lighttransducers include photodiodes and photogate transistors. A variety ofcharge integrating photodiode devices have been described includingthose in U.S. Pat. Nos. 4,916,307 to Nishibe et al.; 5,214,274 to Yang;5,243,215 to Enomoto et al.; 5,338,691 to Enomoto et al.; and 5,789,737to Street. Photogate transistor devices are described in U.S. Pat. Nos.5,386,128 to Fossum et al. and 5,471,515 to Fossum et al. Each of thesepatents is hereby incorporated by reference.

One difficulty with all types of light sensors is the occurrence ofoperating anomalies at high temperatures. Some devices become extremelynon-linear at high temperatures. Some devices, such as CdS cells, maysuffer a permanent change in operating characteristics. Devices may evenprovide completely false readings such as indicating bright light in lowlight conditions due to excessive thermal noise. Traditionally, the onlyway to deal with this problem has been to incorporate a temperaturesensor and associated electronics into the automatic dimming mirror.

What is needed is an automatic dimming mirror that derives the benefitsprovided by semiconductor light sensors with integral charge collection.The mirror should operate over a wide range of lighting conditions andbe less susceptible to temperature variations.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an automaticallydimming mirror capable of operating over a wide range of lightingconditions.

Another object of the present invention is to provide an automaticallydimming mirror utilizing charge integrating semiconductor light sensors.

Still another object of the present invention is to provide anautomatically dimming mirror with less susceptibility to temperaturevariations.

Yet another object of the present invention is to provide anautomatically dimming mirror that is inexpensive to produce.

A further object of the present invention is to provide an automotiverearview mirror system with improved exterior mirror performance.

In carrying out the above objects and other objects and features of thepresent invention, a rearview mirror for an automotive vehicle allowinga vehicle operator to view a scene generally behind the vehicle operatoris provided. The mirror includes a dimming element having a variablyreflective surface. The degree of reflectivity is based on signals froman ambient light sensor positioned to receive light from a regiongenerally in front of the vehicle and from a glare sensor positioned toview the scene generally behind the vehicle operator. At least one ofthe ambient light sensor and the glare sensor accumulates charge inresponse to incident light over a variable integration period.

In another rearview mirror, the ambient light sensor outputs a discreteambient light signal based on the amount of light incident on theambient light sensor over an ambient light integration period. The glaresensor outputs a discrete glare signal based on the amount of lightincident on the glare sensor over a glare integration period. Dimminglogic determines an ambient light level based on the ambient lightsignal. The glare signal resulting from the glare integration perioddetermines a mirror glare level used to set the dimming element controlsignal.

In an embodiment of the present invention, the ambient light sensor hasan input for receiving an ambient light integration period signalspecifying the ambient light integration period. The dimming logicoutputs the ambient light integration period signal based on at leastone previously determined ambient light level.

In another embodiment of the present invention, the dimming logicgenerates a sequence of ambient light integration period signals, eachintegration period signal specifying a different light integrationperiod. The ambient light level is determined based on a resultingambient light signal having a pulse width within at least one presetwidth threshold.

In still another embodiment of the present invention, at least one ofthe ambient light sensor and the glare sensor is a light sensorincluding an exposed light transducer which accumulates charge inproportion to light incident over an integration period. Sensor logicdetermines the light integration period prior to beginning integration.Charge accumulated in the exposed light transducer is reset at thebeginning of the light integration period. Charge accumulated by theexposed light transducer over the light integration period is measuredand a pulse having a width based on the measured accumulated charge isoutput.

In yet another embodiment of the present invention, the light transducerfurther includes a light transducer shielded from ambient light. Theshielded light transducer accumulates charge in proportion to noise overthe integration period. Sensor logic resets the charge accumulated inthe shielded light transducer at the beginning of the light integrationperiod. The charge accumulated by the shielded light transducer over theintegration period is measured and a pulse having a width based on thedifference between the measured accumulated exposed light transducercharge and the measured accumulated shielded light transducer charge isoutput.

In a further embodiment of the present invention, the light sensorreceives an integration pulse having a width determining the integrationperiod. The output pulse is generated after the integration pulse isreceived. The difference in time between the end of the integrationpulse and the start of the output pulse indicates the amount of thermalnoise in the light sensor. In a refinement, the rearview mirror dimminglogic determines the amount of time between the end of the integrationpulse and the start of the output pulse. The light sensor temperature isdetermined based on the elapsed time. The dimming logic may disableautomatic dimming of the dimming element reflective surface if the lightsensor temperature exceeds a preset limit.

In a still further embodiment of the present invention, at least one ofthe ambient light sensor and the glare sensor is a light transducerincluding an enclosure having a window for receiving ambient light. Theenclosure admits a power pin, a ground pin, and a signal pin. An exposedlight transducer within the enclosure accumulates charge in proportionto light received through the window and incident on the exposed lighttransducer. A light-to-pulse circuit within the enclosure outputs apulse having a pulse width based on charge accumulated by the exposedlight transducer over an integration period. Sensor logic within theenclosure receives an integration pulse on the signal pin, determinesthe integration period based on the width of the integration pulse, andoutputs the output pulse on the signal pin.

In yet a still further embodiment of the present invention, the dimminglogic includes at least one signal pin, each signal pin connected to thesignal pin of the ambient light sensor or the glare sensor. For eachdimming logic signal pin, the dimming logic sets the signal pin tooutput mode. An integration period is determined and an integrationpulse generated on the signal pin. The signal pin is set to input mode.The light sensor output pulse is received and a light level determinedbased on the light sensor output pulse duration.

In an additional embodiment of the present invention, a glare lens isprovided for the glare sensor. The glare lens provides the glare sensorwith a narrower field of view and a higher optical gain than the ambientlight sensor.

A rearview mirror system for an automotive vehicle is also provided. Therearview mirror system includes an interior rearview mirror and at leastone exterior rearview mirror. In one embodiment, each mirror contains adimming element, an ambient light sensor, a glare sensor, and a dimminglogic. The dimming logic determines the glare integration period basedon the ambient light level and uses the resulting glare level todetermine the dimming element control signal. In another embodiment,each exterior mirror contains a glare sensor but no ambient lightsensor. Each exterior mirror determines the glare integration periodbased on an ambient light signal from the interior rearview mirror. Eachexterior mirror determines a dimming control value based on glare levelobtained from the exterior mirror glare sensor. In still anotherembodiment, exterior mirrors contain no light sensors and receive asignal indicating the degree of reflectivity for exterior dimmingelements from the interior rearview mirror.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing illustrating vehicle rearview mirrors that mayincorporate the present invention;

FIG. 2 is a block diagram of an embodiment of the present invention;

FIG. 3 is a timing diagram illustrating integration control and sensoroutput for a light sensor that may be used to implement the presentinvention;

FIG. 4 is a schematic diagram of circuitry permitting dimming logic anda light sensor to be interconnected by a single line carrying bothintegration control and sensor output;

FIG. 5 is a timing diagram illustrating operation of the circuitry ofFIG. 4;

FIG. 6 is a block diagram of a rearview mirror system with interior andexterior rearview mirrors according to embodiments of the presentinvention;

FIG. 7 is a schematic diagram illustrating an embodiment of the dimminglogic;

FIG. 8 is a schematic diagram illustrating operation of electrochromicelement transmittance control;

FIG. 9 is a timing diagram illustrating electrochromic elementtransmittance control;

FIG. 10 is a graph indicating dimmer reflectance as a function of dimmercontrol signal duty cycle;

FIG. 11 is a flow diagram illustrating operation of dimming logicaccording to an embodiment of the present invention;

FIG. 12 is a graph illustrating binary logarithmic approximationimplemented in an embodiment of the dimming logic;

FIG. 13 is a schematic diagram illustrating operation of a light sensorhaving a pulse output according to an embodiment of the presentinvention;

FIG. 14 is a timing diagram illustrating operation of the light sensorof FIG. 13;

FIG. 15 is a schematic diagram illustrating operation of a light sensorwith noise compensation according to an embodiment of the presentinvention;

FIG. 16 is a timing diagram illustrating operation of the light sensorof FIG. 15;

FIG. 17 is a schematic diagram of an implementation of the light sensorof FIG. 15 using photodiodes as light transducers;

FIGS. 18-21 are block diagrams of various embodiments for light sensorpackaging, output, and control;

FIG. 22 is a block diagram of sensor logic for internally determiningthe integration period signal;

FIG. 23 is a block diagram illustrating the use of light transducershaving different effective areas to achieve increased dynamic rangeaccording to an embodiment of the present invention;

FIG. 24 is a block diagram illustrating the use of light transducershaving different apertures to achieve increased dynamic range accordingto an embodiment of the present invention;

FIG. 25 is a schematic diagram illustrating different transducercapacitances for different amounts of light-induced charge to achieveincreased dynamic range according to an embodiment of the presentinvention;

FIG. 26 is a graph of the output potential as a function of accumulatedincident light for the transducer of FIG. 25;

FIG. 27 is a schematic diagram illustrating a photodiode transducerincorporating an anti-bloom gate according to an embodiment of thepresent invention;

FIG. 28 is a drawing illustrating an enclosure for a light sensoraccording to an embodiment of the present invention;

FIG. 29 is a graph illustrating light sensor field of view as a functionof light transducer distance from the lens;

FIG. 30 is a graph illustrating light sensor optical gain as a functionof light transducer distance from the lens;

FIG. 31 is a graph illustrating frequency response of the human eye;

FIG. 32 is a graph illustrating frequency response of a typical lighttransducer; and

FIG. 33 is a drawing of an enclosure incorporating an infrared filteraccording to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, a drawing illustrating vehicle rearview mirrorsthat may incorporate the present invention is shown. Vehicle 20 isdriven by operator 22. Operator 22 uses interior rearview mirror 24 andone or more exterior rearview mirrors 26 to view a rearward scene, showngenerally by 28. Most of the time, operator 22 is looking forwardthrough windshield 30. The eyes of operator 22 therefore adjust toambient light 32 coming from a generally forward direction. A relativelybright light source in rearward scene 28 may produce light which canreflect from mirrors 24,26 to temporarily visually impair, distract, ordazzle operator 22. This relatively strong light is known as glare 34.

To reduce the impact of glare 34 on operator 22, the reflectance ofmirrors 24,26 may be reduced. Prior to automatically dimming mirrors,interior rearview mirror 24 would contain a prismatic reflective elementthat could be manually switched by operator 22. Automatically dimmingmirrors include a sensor for glare 34 and, typically, for ambient light32, and dim one or more mirrors 24,26 in response to the level of glare34.

Referring now to FIG. 2, a block diagram of an embodiment of the presentinvention is shown. A dimming element, shown generally by 40, includesvariable transmittance element 42 and reflective surface 44. Dimmingelement 40 is positioned such that reflective surface 44 is viewedthrough variable transmittance element 42. Dimming element 40 exhibitsvariable reflectance of light in response to dimming element controlsignal 46. Ambient light sensor 48 is positioned to receive ambientlight 32 from generally in front of vehicle 20. Ambient light sensor 48produces discrete ambient light signal 50 indicating the amount ofambient light 32 incident on ambient light sensor 48 over an ambientlight integration period. Glare sensor 52 is positioned to detect glare34 from generally behind vehicle 20 and may optionally be placed to viewglare 34 through variable transmittance element 42. Glare sensor 52produces discrete glare signal 54 indicating the amount of glare 34incident on glare sensor 52 over a glare integration period. Dimminglogic 56 receives ambient light signal 50 and determines an ambientlight level. Dimming logic 56 determines the glare integration periodbased on the level of ambient light 32. Dimming logic 56 receives glaresignal 54 and determines the level of glare 34. Dimming logic 56 outputsdimming element control signal 46, setting the reflectance of dimmingelement 40 to reduce the effects of glare 34 perceived by operator 22.

Either glare sensor 52, ambient light sensor 48 or, preferably, both aresemiconductor light sensors with integral charge collection. Suchsensors include light transducers which convert incident light intocharge. This charge is collected over an integration period to produce apotential which is converted by sensor 48,52 into a discrete output.Designs suitable for this application are described in U.S. Pat. No.4,916,307 entitled “Light Intensity Detecting Circuit With Dark CurrentCompensation” by Nishibe et al.; and U.S. Pat. No. 5,214,274 entitled“Image Sensor Array With Threshold Voltage Detectors And Charged StorageCapacitors” to Yang; each of which is incorporated herein by reference.Preferred embodiments for light sensors 48,52 are described with regardto FIGS. 13-33 below.

One difficulty with silicon-based sensors is the difference in spectralsensitivity between silicon and the human eye. Ambient light filter 58may be placed before or incorporated within ambient light sensor 50.Similarly, glare filter 60 may be placed before or incorporated withinglare sensor 52. Filters 58,60 attenuate certain portions of thespectrum that may include visible light, infrared, and ultravioletradiation such that light striking sensors 48,52 combines with thefrequency response of light transducers within sensors 48,52 to moreclosely approximate the response of the human eye and to compensate fortinting in vehicle windows such as windshield 30. The use of filters58,60 to compensate for the spectral sensitivity of light transducerswithin sensors 48,52 is described with regards to FIGS. 31-33 below.

Variable transmittance element 42 may be implemented using a variety ofdevices. Dimming may be accomplished mechanically as described in U.S.Pat. No. 3,680,951 titled “Photoelectrically-Controlled Rear-ViewMirror” to Jordan et al.; and U.S. Pat. No. 4,443,057 titled “AutomaticRearview Mirror For Automotive Vehicles” to Bauer et al.; each of whichis incorporated herein by reference. Variable transmittance element 42may be formed using liquid crystal cells as is described in U.S. Pat.No. 4,632,509 titled “Glare-Shielding Type Reflector” to Ohmi et al.,which is incorporated herein by reference. Preferably, variabletransmittance element 42 is an electrochromic cell which variestransmittance in response to an applied control voltage such as isdescribed in U.S. Pat. No. 4,902,108 titled “Single-Compartment,Self-Erasing, Solution-Phase Electrochromic Devices, Solutions For UseTherein, And Uses Thereof” to Byker, which is incorporated herein byreference. Many other electrochromic devices may be used to implementdimming element 40, some of which are mentioned in the Background Artsection of this application. As will be recognized by one of ordinaryskill in the art, the present invention does not depend on the type orconstruction of dimming element 40. If dimming element 40 includeselectrochromic variable transmittance element 42, reflective surface 44may be incorporated into variable transmittance element 42 or may beexternal to variable transmittance element 42.

Each interior rearview mirror 24 and exterior rearview mirror 26 mustinclude dimming element 40 for automatic dimming. Preferably, interiorrearview mirror 24 also includes dimming logic 56, light sensors 48,52,and, if used, filters 58 and 60. Various embodiments for controllingexterior rearview mirrors 26 are described with regard to FIG. 6 below.

Referring now to FIG. 3, a timing diagram illustrating integrationcontrol and sensor output for a light sensor that may be used toimplement the present invention is shown. Charge accumulating lightsensors 48,52 exhibit increased dynamic range through variableintegration periods. A control signal, shown generally by 70, is used tospecify the integration period. The resulting sensor output, showngenerally by 72, includes an output pulse for each integration period.The total amount of light-induced charge which can be effectivelymeasured is limited. Therefore, in the presence of bright light, a shortintegration time is desirable to prevent saturation. However, if a shortintegration time is used in low light conditions, the charge signal maybe lost in noise inherent in sensor 48,52.

Control signal 70 includes a sequence of integration periods havingvarying lengths. In the example shown in FIG. 3, short integration pulse74 having short integration period 76 is generated. A semiconductorlight sensor may output a short pulse in a completely dark environmentdue to noise. Therefore, any pulse in sensor output 72, such as shortsignal pulse 78, having a duration less than a threshold is ignored.Next, medium integration pulse 80 having medium integration period 82 isgenerated. Resulting medium signal pulse 84 has a duration indicative ofthe amount of light incident on sensor 48,52 during medium integrationperiod 82. Long integration pulse 86 having long integration period 88is generated. If light sensor 48,52 is sufficiently bright, saturationwill result. Therefore, long signal pulse 90 having a duration greaterthan a threshold is also ignored.

Control signal 70 may be generated outside of light sensor 48,52 or maybe generated by control logic within light sensor 48,52. If generatedexternally, control signal 70 and sensor output 72 may share a commonsignal line or may use separate signal lines. Various options andembodiments are described with regard to FIGS. 4-28 below.

Referring now to FIG. 4, a schematic diagram of circuitry permittingdimming logic and a light sensor to be interconnected by a single linecarrying both integration control and sensor output is shown. Lightsensor 48,52 includes enclosure 100 with window 102 admitting light 104incident on exposed light transducer 106. Enclosure 100 admits power pin108, ground pin 110, and signal pin 112. The use of only three pins108,110,112 greatly reduces the cost of light sensor 48,52. A three-pinpackage that may be used to implement light sensor 48,52 is describedwith regards to FIG. 28 below.

Light sensor 48,52 is connected to dimming logic 56 throughinterconnection signal 114 between signal pin 112 in light sensor 48,52and signal pin 116 in dimming logic 56. As will be described below,signal pins 112,116 are tri-state ports permitting interconnect signal114 to provide both an input to light sensor 48,52 and an output fromlight sensor 48,52. Dimming logic 56 may include FET Q1 connectedbetween signal pin 116 and ground. FET Q1 is controlled by control line118 connected to the base of Q1. Buffer 120 is also connected to signalpin 116.

Within light sensor 48,52, FET Q2 is connected between signal pin 112and ground. FET Q2 is controlled by output pulse 122 connected to thegate of Q2. Constant current source 124 is connected to signal pin 112so that if neither Q1 or Q2 are on, interconnect signal 114 is pulledhigh. Constant current source 124 nominally sources about 0.5 mA to pullup interconnect signal 114. The input of Schmidt trigger inverter 126 isconnected to signal pin 112. Schmidt trigger inverter 126 is followed byinverters 128 and 130 in series. The output of inverter 130 clocks Dflip-flop 132. The output of multiplexer 134 is connected to the D inputof flip-flop 132. The select input of multiplexer 134 is driven byoutput pulse 122 such that when output pulse 122 is asserted the D inputof flip-flop 134 is unasserted and when output pulse 122 is not assertedthe D input of flip-flop 134 is asserted. The output of NAND gate 136 isconnected to low asserting reset 138 of flip-flop 132. The output offlip-flop 132 is integration pulse 140. Integration pulse 140 and theoutput of inverter 128 are inputs to NAND gate 136. Light-to-pulsecircuit 142 accepts integration pulse 140 and the output of exposedlight transducer 106 and produces output pulse 122. Embodiments forlight-to-pulse circuit 142 are described with regard to FIGS. 13-17 and23-27 below.

In a preferred embodiment, light sensor 48,52 includes shielded lighttransducer 144 which does not receive light 104. Light-to-pulse circuit142 uses the output of shielded light transducer 144 to reduce theeffects of noise in exposed light transducer 106.

Referring now to FIG. 5, a timing diagram illustrating operation of thecircuitry of FIG. 4 is shown. Initially, low asserting interconnectsignal 114 is high. The state of flip-flop 132 must be zero for, if thestate is one, both inputs to NAND gate 136 would be high, assertingreset 138 and forcing the state of flip-flop 132 to zero.

At time 150, dimming logic 56 asserts control line 118 turningtransistor Q1 on. Interconnect signal 114 is then pulled low at time152. The output of inverter 130 transitions from low to high setting thestate of flip-flop 132 to one which causes integration pulse 140 tobecome asserted at time 154. Light-to-pulse circuit 142 beginsintegrating light 104 incident on exposed light transducer 106. At time156, control line 118 is brought low turning transistor Q1 off. Thedifference between time 156 and time 150 is integration period 158requested by dimming logic 56. Since both Q1 and Q2 are off,interconnect signal 114 is pulled high by current source 124 at time160. Since the output of inverter 128 and integration pulse 140 are bothhigh, reset 138 is asserted causing the state of flip-flop 132 to changeto zero and integration pulse 140 to become unasserted at time 162. Thissignals light-to-pulse circuit 142 to stop integrating light 104incident on exposed light transducer 106.

At time 164, light-to-pulse circuit 142 asserts output pulse 122 tobegin outputting light intensity information. Asserting output pulse 122turns transistor Q2 on, pulling interconnect signal 144 low at time 166.This causes inverter 130 to output a low-to-high transition clocking azero as the state of flip-flop 132. Light-to-pulse circuit 142 deassertsoutput pulse 122 at time 168. The difference between time 168 and time164 is light intensity period 170 indicating the amount of light 104incident on exposed light transducer 106 over integration period 158.Transistor Q2 is turned off when output pulse 122 goes low at time 168.Since both transistors Q1 and Q2 are off, interconnect signal 114 ispulled high at time 172. Buffer 120 in dimming logic 56 detects thetransitions in interconnect signal 114 at times 166 and 172. Thedifference in time between times 172 and 166 is used by dimming logic 56to determine the intensity of light 104 received by light sensor 48,52.

If shielded light transducer 144 is included in light sensor 48,52, thedifference in time between the deassertion of integration pulse 140 attime 162 and the assertion of output pulse 122 at time 164 is due, inpart, to the thermal noise in light sensor 48,52. This difference isexpressed as thermal noise period 174. Thermal noise period 174 may beused by dimming logic 56 to determine the temperature of light sensor48,52 or may be more simply used to determine if the noise level insensor 48,52 is too high for a reliable reading. Dimming logic 56 maydisable automatic dimming of dimming element 40 if the temperature oflight sensor 48,52 exceeds a preset limit. The ability of light sensor48,52 to use the output from shielded light transducer 144 to generateoutput pulse 122 indicative of the amount of thermal noise in lightsensor 48,52 is described with regard to FIGS. 15-17 below.

Referring now to FIG. 6, a block diagram of a rearview mirror systemwith interior and exterior rearview mirrors according to embodiments ofthe present invention is shown. Dimming element 40 in interior rearviewmirror 24 operates as described with regard to FIG. 2 above. Eachexterior rearview mirror 26 includes exterior dimming element 180 havingexterior variable transmittance element 182 operative to attenuate lightfrom rearward scene 28 both prior to and after reflecting from exteriorreflective surface 184. Exterior dimming element 180 provides variablereflectance based on exterior dimming element control signal 186.Exterior dimming element 180 may operate in any manner described withregard to dimming element 40 and, preferably, is an electrochromicmirror. Exterior mirror control 188 generates exterior dimming elementcontrol signal 186. Exterior mirror control 188 may be part of exteriorrearview mirror 26, interior rearview mirror 24, or may be locatedoutside of any mirror 24,26. Various embodiments for controllingexterior dimming element 180 depend on the amount of sensing and controlto be included within exterior rearview mirror 26.

In one embodiment, dimming logic 56 in interior rearview mirror 24determines exterior dimming element control signal 186 based on outputfrom ambient light sensor 48 and glare sensor 52. Exterior dimmingelement control signal 186 may be generated directly by dimming logic 56or exterior mirror control 188 may generate exterior dimming elementcontrol signal 186 based on a reflectance level calculated in dimminglogic 56 and transmitted to exterior mirror control 188 throughinter-mirror signal 190.

In another embodiment, exterior rearview mirror 26 includes exteriorglare sensor 192 positioned to receive glare 34 from rearward scene 28and operative to output exterior glare signal 194 based on the amount ofglare 34 incident on glare sensor 192 over a glare integration period.Since light sensors 48,52 with silicon-based charge accumulatingtransducers 106,144 have a lower cross-sensitivity to temperature,mounting light sensors 48,52 outside the body of vehicle 20 is morepractical than with other types of light transducers. Dimming logic 56uses exterior glare signal 194 and ambient light signal 50 to determinethe reflectance level for exterior dimming element 180. Again, exteriordimming element control signal 186 may be generated directly by dimminglogic 56 or may be developed by exterior mirror control 188 based on thereflectance level contained in inter-mirror signal 190. Exterior glarefilter 196, similar to glare filter 60, may be placed before exteriorglare sensor 192 or built into exterior glare sensor 192 to provideexterior glare sensor 192 with a response closer to the response of thehuman eye. Inter-mirror signal 190 and exterior glare signal 194 may bein the form of a pulse width modulated signal, pulse density signal,serial data stream, or digitized and communicated over an automotive bussuch as the CAN bus.

In still another embodiment, exterior glare sensor 192 produces exteriorglare signal 198 which is routed directly to exterior mirror control188. Exterior mirror control 188 determines exterior dimming elementcontrol signal 186 based on exterior glare signal 198 and the level ofambient light 32 determined by dimming logic 56 and sent to exteriormirror control 188 through inter-mirror signal 190.

In yet another embodiment, exterior rearview mirror 26 determinesreflectance for exterior dimming element 180 independent of glare 34 orambient light 32 sensed by interior rearview mirror 24. In thisembodiment, exterior rearview mirror 26 operates as described withregard to FIG. 2 above.

Referring now to FIG. 7, a schematic diagram illustrating an embodimentof the dimming logic is shown. The circuit represents an effective yetinexpensive implementation for automatically dimming interior rearviewmirror 24. Dimming logic 56 utilizes a small, low cost microcontroller,indicated by U1, such as the PIC 16C620 from Microchip Technology, Inc.of Chandler, Ariz. Ambient light sensor 48 communicates withmicrocontroller U1 through interconnection signal 114 connected tomicrocontroller input RB0. Similarly, glare sensor 52 communicates withmicrocontroller U1 through separate interconnection signal 114 aconnected to microcontroller input RB2. As described with regard toFIGS. 4 and 5 above, each interconnection signal 114 carries integrationperiod 158 from microcontroller U1 to light sensor 48,52 as well aslight intensity period 170 from light sensor 48,52 to microcontrollerU1. Resistor R29 and capacitor C4 connected between V_(DD) and groundprovide filtered power for light sensors 48,52.

Parallel resistor R15 and diode D5 are connected between V_(DD) and node208. Capacitor C12 is connected between node 208 and ground. Resistor R6connects common node 208 to input /MCLR of microcontroller U1.Components D5, R15, R6, and C12 form a power-on reset circuit showngenerally by 210.

Power is supplied to dimming logic 56 through ignition line 212. DiodeD1 protects from reversed polarity on ignition line 212 and diode D2clamps the voltage derived from ignition line 212 to approximately fivevolts. Capacitors C2, C7 and C11, resistor R3, and ferrite element Elform a power conditioning circuit shown generally by 214. Reverse line216 is asserted when vehicle 20 is placed into reverse. Capacitor C10and resistors R8, R9, and R27 form a reverse signal conditioningcircuit, shown generally by 218. Reverse signal conditioning circuit 218low pass filters reverse line 216 and provides electrostatic dischargeprotection for digital input pin RB6 on microcontroller U1.Microcontroller U1 uses the signal on reverse line 216 to clear variabletransmittance element 42 whenever vehicle 20 is placed in reverse.Microcontroller U1 is clocked by an RC oscillator formed by resistor R2connected between the OSC1 pin and V_(DD) and capacitor C1 connectedbetween the OSC1 pin and ground. Resistor R30 and LED D3 connected inseries between V_(DD) and open drain output RA4 of microcontroller U1form an indicator lamp that may be mounted on interior rearview mirror24 to alert operator 22 of the operating state of dimming logic 56.Switches S1 and S2 are connected to digital inputs RB1 and RB3,respectively, of microcontroller U1 to permit selecting control options.

Referring now to FIG. 8, a schematic diagram illustrating operation ofelectrochromic dimmer control is shown. A portion of dimmer logic 56 hasbeen redrawn to more clearly illustrate control of electrochromicvariable transmittance element 42. Electrochromic variable transmittanceelement 42 is preferably similar in operation to those described in U.S.Pat. No. 4,902,108 titled “Single-Compartment, Self-Erasing,Solution-Phase Electrochromic Devices, Solutions For Use Therein, AndUses Thereof” to Byker, which is incorporated herein by reference.Electrochromic variable transmittance element 42 darkens in response toa control voltage applied at input node 220. If the applied controlvoltage is removed, electrochromic variable transmittance element 42will self discharge, transmitting an increasing amount of light.Electrochromic variable transmittance element 42 may be rapidly clearedby shorting input node 220 to ground. While the application describedpertains particularly to automotive rearview mirrors, it will beunderstood by one of ordinary skill in the art that all or part ofdimmer logic 56 may be used in a wide variety of electrochromic mirrorand window applications.

Resistor R17 connects input node 220 to the emitter of Darlington pairQ10 at node 222. The collector of Q10 is connected to a power supplythrough current limiting resistor R5, typically 27 Ω. The base ofDarlington pair Q10 is connected to digital output RB4 ofmicrocontroller U1 through resistors R1 and R7. The base of Q10 is alsoconnected to ground through resistor R4 and through resistor R7 andcapacitor C16. Digital output pin RB4 is driven by pulse output 224 inresponse to pulse control 226 generated by software running onmicrocontroller U1. Pulse output 224 may produce a pulse signal such as,for example, a pulse width modulated signal. Preferably, pulse output224 functions as a switch, setting output pin RB4 to either a highvoltage or a low voltage once during each transition period as describedwith regards to FIG. 9 below. Capacitor C16 and resistors R1, R4, and R7form a low pass filter, shown generally by 227, to smooth the signalappearing on digital output RB4. This smoothing results in asubstantially constant applied control voltage at input node 220 for afixed desired control level. Additionally, the base-to-emitter diodedrops in Q10 together with the voltage divider formed between resistorR4 and the sum of resistors R1 and R7 set the operating voltage forelectrochromic variable transmittance element 42. Typical values forcomponents are 1 kΩ for R1 and R4, 100 Ω for R7, and 100 μF for C16.With digital output RB4 at 5 volts and nominal current draw byelectrochromic variable transmittance element 42, input node 220 isapproximately 1.2 volts.

The performance of dimming logic 56 can be improved through feedback ofelectrochromic variable transmittance element 42 applied control voltageat input node 220. Microcontroller U1 includes comparison logic to causepulse output 224 to deliver a low voltage if the applied control voltageis greater than the desired control level and to deliver a high voltageotherwise. Typically, the high voltage is near V_(DD) and the lowvoltage is near ground. This comparison may be made by comparing adigital number representing the desired control level with the digitizedapplied control voltage obtained using an analog-to-digital converter.Alternately, digital-to-analog converter (DAC) 228 and comparator 230are used. DAC 228 produces a desired voltage level on analog output AN2in response to the desired control level on DAC control 232 supplied bysoftware running on microcontroller U1. Resistor R31 is connectedbetween analog output AN2 and node 234 and resistor R26 is connectedbetween node 234 and ground. One input of comparator 230, at analoginput AN3, is connected to node 234. The other input of comparator 230,at analog input AN0, is connected to input node 220. The output ofcomparator 230 indicates if the desired voltage level is greater thanthe applied control voltage. Values for resistors R31 and R26 are chosenso that the voltage at node 234 is within the range of expected appliedcontrol voltages at input node 220 throughout the range of desiredcontrol voltages output from DAC 228. Typical values for R31 and R26 are390 kΩand 200 kΩ, respectively.

Positive feedback is achieved by connecting resistor R24 between node234 and node 222. Resistor R17 is used to sense the drive currentthrough electrochromic variable transmittance element 42 and, hence, istypically a low value such as 10 Ω. Resistor R24 is typically a highvalue such as 1.3 MΩ. As the drive current through resistor R17increases, the voltage across resistor R17 increases pulling up thevoltage at node 234. This increase in the voltage on the positive inputterminal of comparator 230 has the regenerative effect of increasing theduty cycle from pulse output 224. This regenerative effect providesbetter system response at higher temperatures when electrochromicvariable transmittance element 42 has an increased current draw togetherwith an increase in maximum operating voltage. Positive feedback alsooffsets the effects of internal resistances within electrochromicvariable transmittance element 42.

Referring now to FIG. 9, a timing diagram illustrating electrochromicelement transmittance control is shown. During automatic dimmingoperation, software executing in microcontroller U1 is initiated attransition points, one of which is indicated by 240, separated by fixedtransition period 242. Desired control level 244 indicates the desiredlevel of transmittance for electrochromic variable transmittance element42. Desired control level 244 may be an analog value or, preferably, isa digital number determined by microcontroller U1. Desired control level244 is compared to applied control voltage 246 by comparison logic.Comparator 230 accepts applied control voltage 246 and the desiredcontrol voltage appearing at node 234. Comparator output 236 producesdifference signal 248 which is asserted when the desired voltage levelrepresenting desired control level 244 is greater than applied controlvoltage 246. Comparator output 236 is used to generate control signal250 on output RB4. If desired control level 244 is greater than appliedcontrol voltage 246, digital output RB4 is switched high. If desiredcontrol level 244 is less than applied control voltage 246, digitaloutput RB4 is switched low. Preferably, low pass filter 227 filterscontrol signal 250 to produce applied control voltage 246.

The duration of transition period 242 is set to inhibit flicker inelectrochromic element 42 that may be noticed, for example, by vehicleoperator 22. Transition period 242 may preferably be between two secondsand two microseconds. For the system described with regards to FIGS. 7and 8 above, five milliseconds may be used for transition period 242.

Referring now to FIG. 10, a graph indicating dimmer reflectance as afunction of applied control voltage is shown. Curve 254 plots reflectionas a percentage for dimming element 40 containing electrochromicvariable transmittance element 42 as a function of applied controlvoltage 256. Curve 254 indicates a decrease in reflection from about 86%to about 8% as the applied control voltage is increased from about 0.2 Vto about 0.9 V. FIG. 10 also includes curve 256 illustrating currentdraw as a function of applied control voltage 256 for typicalelectrochromic variable transmittance element 42.

Referring again to FIG. 7, additional circuitry is provided to rapidlyclear variably transmissive electrochromic element 40. Transistor Q11 isconnected across variably transmissive electrochromic element 40 withcollector at node 220 and emitter at ground. The base of transistor Q11is connected through resistor R23 to digital output RB7. When digitaloutput RB7 is asserted, transistor Q11 turns on, acting as a switch torapidly discharge electrochromic variable transmittance element 42.Capacitor C6 is connected between the collector and base of transistorQ11 to reduce electromagnetic interference created as transistor Q11switches. Transistor Q12 is connected between the base of transistor Q10and ground and is also controlled by digital output RB7. Transistor Q11turns on with transistor Q12 to shut off transistor Q10 therebypreventing simultaneously attempting to darken and clear electrochromicvariable transmittance element 42. Resistor R7 is placed betweencapacitor C16 and the collector of transistor Q12 to limit the dischargecurrent from capacitor C16 through transistor Q12.

Referring now to FIG. 11, a flow diagram illustrating operation ofdimming logic according to an embodiment of the present invention isshown. As will be appreciated by one of ordinary skill in the art, theoperations illustrated are not necessarily sequential operations. Also,though the operations are preferably implemented by software executingin microcontroller U1, operations may be performed by software,hardware, or a combination of both. The present invention transcends anyparticular implementation and aspects are shown in sequential flow chartform for ease of illustration.

An ambient light reading is taken and the average ambient light isinitialized in block 260. When the automatic dimming system is initiallypowered up, the average ambient light level is initialized by taking afirst reading of ambient light 32 using ambient light sensor 48.Acquiring an ambient light reading and the average ambient light levelare described with regard to blocks 262 and 270, respectively, below.

An ambient light reading is taken and the log of the ambient lightreading is found in block 262. The use of semiconductor ambient lightsensor 48 with integral charge collection produces ambient light signal50 having good resolution over a wide range of ambient light levels 32.As described with regard to FIG. 3 above, this is accomplished by takingvarious readings of ambient light 32 using different integration periods76,82,88. In a preferred embodiment, four separate integration periodsare used such as, for example, 600 μs, 2.4 ms, 9.6 ms, and 38.4 ms. Eachof these integration periods differs by a factor of four from adjacentperiods. Therefore, for example, the 2.4 ms integration period causesambient light sensor 48 to act four times more sensitive to ambientlight 32 than does integrating with the 600 μs integration period.Typically, the shortest integration pulse 74 is first used by ambientlight sensor 48 to produce short signal pulse 78.

The width of short signal pulse 78 is measured by dimming logic 56.Since ambient light sensor 48 in complete darkness may still developshort signal pulse 78 having a width less than 100 μs, a minimumthreshold is set for accepting short signal pulse 78 as accuratelyreflecting the level of ambient light 32. Typically, this threshold maybe 300 μs. If short signal pulse 78 does not exceed the threshold, thenext longest integration period is used by ambient light sensor 48. Ifthe longest integration time does not yield a suitably long signalpulse, ambient light 32 is at an extremely low level and mirror 24,26can be operated at maximum sensitivity to glare 34.

Using the logarithm of ambient light signal 50 permits the use of aninexpensive microcontroller such as U1 which may have only 8-bitinternal registers and no multiplication instructions. Sincemicrocontrollers are binary devices, base two logarithms require fewerinstructions to compute than base ten logarithms or natural logarithms.An algorithm is now described for obtaining an 8-bit, binary logarithmhaving the most significant four bits representing an integer part andthe least significant four bits a fractional part. The 8-bit ambientlight signal 50 resulting from the proper integration period is examinedbit-by-bit starting with the most significant bit until the first binaryone is found. The bit position containing the first binary one becomesthe integer portion of the logarithm. The four most significant bitsfollowing the bit position containing the first binary one become thefractional portion of the logarithm. This value is incremented byone-sixteenth to better approximate the logarithm. An example of thebinary logarithm approximation is now provided. Suppose ambient lightsignal 50 is determined to be 44 (00101101 in base two). The mostsignificant asserted bit is bit five, so the integer portion of theresultant value is binary 0101. The next four bits following bit fiveare 0110 so the fractional part of the resultant value is 0110 for atotal value of 0101.0110. After incrementing, the binary logarithmapproximation becomes 0101.0111.

Referring now to FIG. 12, a graph illustrating binary log approximationaccording to the above algorithm is shown. The binary logarithm isplotted for values of N between 1 and 255. Curve 290 shows the actualbinary logarithm. Curve 292 shows the approximated binary logarithm.

Ambient light signal 50 must be scaled to compensate for differentpossible integration periods. This may be accomplished by adding ascaling factor to the binary logarithm of ambient light signal 50. Forexample, if the longest integration time (38.4 ms) is used to measureambient light 32, a scale factor of 0 is added. If the next longestintegration time (9.6 ms) is used, a scale of factor of 2 is added. Ifthe next longest integration time (2.4 ms) is used, 4 is added. If theshortest integration time (600 μs) is used, 6 is added. Since thelargest value resulting from the binary logarithm approximation is 8(1000.0000), no overflow results from adding the scale factor.

Referring again to FIG. 11, the logarithm of the ambient light level iscompared to the day detect level in block 264. The day detect level isused to prevent dimming of or to more rapidly clear dimming element 40during rapid transitions from dark to bright such as if vehicle 20emerges from a tunnel into daylight. If the logarithm of ambient light32 exceeds a preset day detect level, variable transmittance element 42is cleared to set dimming element 40 to maximum reflectance in block266. Processing is then delayed in block 268. A wait loop is enteredhaving a time sufficiently long to make the period between takingambient light readings equal a constant ambient light loop delay. Thisperiod may be, for example, 400 ms. Following the wait in block 268,another reading of ambient light 32 is taken in block 262. If thelogarithm of ambient light 32 does not exceed the day detect level, anaverage is obtained in block 270.

The average of the logarithm of ambient light level is determined inblock 270. Averaging readings first converted to the logarithm ofambient light 32 reduces the effect of a temporary bright light in frontof vehicle 20 from dramatically skewing the average reading of anotherwise dark ambient light 32. A running average of the log of ambientlight signals 50 may be obtained from a digital low pass filter such asis described by Equation 1: $\begin{matrix}{{y(n)} = {{\frac{1}{64}{x(n)}} + {\frac{63}{64}{y( {n - 1} )}}}} & (1)\end{matrix}$

where x(n) is the most recently obtained binary log approximation ofambient light signal 50 correctly scaled for the integration period,y(n−1) is the previous filter output, and y(n) is the current filteroutput. The use of averaged logarithms with analog light signals isdescribed in U.S. Pat. No. 5,204,778 titled “Control System ForAutomotive Rearview Mirrors” to Bechtel, which is incorporated herein byreference.

The average of the log of the ambient light level is compared to athreshold in block 272. If ambient light 32 is sufficiently bright,vehicle operator 22 will not be dazzled by any reasonable amount ofglare 34 allowing mirror 24,26 to be set to maximum reflectance.Therefore, if the average of the log of ambient light signal 50 is notless than the threshold, dimming element 40 is cleared in block 266 andthe wait of block 268 is executed. If the average of the log of ambientlight signals 50 is less than the threshold, glare processing occursbeginning in block 274.

Typically, the threshold used for comparison in block 272 is less thanthe day detect level used in the comparison of block 264.

The glare integration period is determined in block 274. The integrationperiod for glare sensor 52 is determined based on ambient light signal50. The glare integration period is inversely proportional to the binaryantilogarithm of the average of the log of ambient light signal 50 asdescribed by Equation 2:

T _(G)(n)=antilog₂(K ₁ −y(n))−K ₂  (2)

where T_(G)(n) is the integration period for glare sensor 52 for thefilter output at sample time n, K₁ is a multiplicative constant, and K₂is an additive constant. Constants K₁ and K₂ are determinedexperimentally. If the average of the log of ambient light signal 50 isbelow a certain level, a maximum glare sensitivity integration period isused.

A glare count is set in block 276. The glare count indicates the numberof glare readings taken between ambient light readings. The product ofthe glare count and the glare loop delay should equal the time betweentaking ambient light readings. For example, the glare count may be threeand the time between taking glare readings may be 133 ms.

A glare reading is taken in block 278. The pulse width returning fromglare sensor 52 as glare signal 54 is measured for the glare integrationperiod determined in block 274.

The dimming element value is set in block 280. Glare signal 54 is usedto determine desired control level 244 setting the reflectance fordimming element 40. This may be accomplished, for example, through theuse of a look-up table. The precise relationship between the level ofglare 34 and the setting for variable transmittance element 42 dependsupon factors including the construction of mirror 24,26, theconfiguration of vehicle 20, and preferential settings by operator 22.Desired control level 244 may be used to control variable transmittanceelement 42 as described with regards to FIGS. 7-10 above.

A check of the glare count is made in block 282. If the glare count iszero, the next ambient light reading is taken in block 262. If the glarecount is not zero, the glare count is decremented in block 284. A waitloop is then entered in block 286. The glare loop delay period is set sothat glare readings are taken at regular, predetermined intervals.

Referring now to FIG. 13, a schematic diagram illustrating operation ofa light sensor having a pulse output according to an embodiment of thepresent invention is shown. Light-to-pulse circuit 300 includes exposedlight transducer 106 for converting light 104 incident on exposed lighttransducer 106 into charge accumulated in light storage capacitor 304,indicated by C_(SL). Exposed light transducer 106 may be any devicecapable of converting light 104 into charge, such as the photogatesensor described in U.S. Pat. No. 5,471,515 titled “Active Pixel SensorWith Intra-Pixel Charge Transfer” to E. Fossum et al., which isincorporated herein by reference. Preferably, light transducer 106 is aphotodiode such as is described with regards to FIGS. 25 and 26 below.Except as noted, the following discussion does not depend on aparticular type or construction for exposed light transducer 106.

Light-to-pulse circuit 300 operates under the control of sensor logic306. Sensor logic 306 generates reset signal 308 controlling switch 310connected between exposed light transducer output 312 and V_(DD). Sensorlogic 306 also produces sample signal 314 controlling switch 316 betweenexposed light transducer output 312 and light storage capacitor 304. Thevoltage across light storage capacitor 304, light storage capacitorvoltage 318, is fed into one input of comparator 320. The other input ofcomparator 320 is ramp voltage 322 across ramp capacitor 324. Rampcapacitor 324 is in parallel with current source 326 generating currentI_(R). Sensor logic 306 further produces ramp control signal 328controlling switch 330 connected between ramp voltage 322 and V_(DD).Comparator 320 produces comparator output 332 based on the relativelevels of light storage capacitor voltage 318 and ramp voltage 322.Sensor logic 306 may generate reset signal 308, sample signal 314, andramp control signal 330 based on internally generated timing or onexternally generated integration pulse 140 as described with regard toFIGS. 18-21 below.

Referring now to FIG. 14, a timing diagram illustrating operation of thelight sensor of FIG. 13 is shown. A measurement cycle is started at time340 when sample signal 314 is asserted while reset signal 308 isasserted. This closes switch 316 to charge light storage capacitor 304to V_(DD) as indicated by voltage level 342 in light storage capacitorvoltage 318. Reset signal 308 is then deasserted at time 344, openingswitch 310 and beginning integration period 346. During integrationperiod 346, light 104 incident on exposed light transducer 106 generatesnegative charge causing declining voltage 348 in light storage capacitorvoltage 318. At time 350, ramp control signal 328 is asserted closingswitch 330 and charging ramp capacitor 324 so that ramp voltage 322 isV_(DD) as indicated by voltage level 352.

Sample signal 314 is deasserted at time 354, causing switch 316 to open,thereby ending integration period 346. At some time 356 following time354 and prior to the next measurement cycle, reset signal 308 must beasserted closing switch 310. At time 358, ramp control signal 328 isdeasserted opening switch 330. This causes ramp capacitor 324 todischarge at a constant rate through current source 326 as indicated bydeclining voltage 360 in ramp voltage 322. Initially, as indicated byvoltage level 362, comparator output 332 is unasserted because rampvoltage 322 is greater than light storage capacitor voltage 318. At time364, declining voltage 360 in ramp voltage 322 drops below light storagecapacitor voltage 318 causing comparator output 332 to become asserted.Comparator output 322 remains asserted until time 366 when ramp controlsignal 328 is asserted closing switch 330 and pulling ramp voltage 322to V_(DD). The difference between time 366 and time 364, indicated bypulse duration 368, is inversely related to the amount of light 104received by exposed light transducer 106 during integration period 346.

Referring now to FIG. 15, a schematic diagram illustrating operation ofa light sensor with noise compensation according to an embodiment of thepresent invention is shown. A light-to-pulse circuit, shown generally by380, improves upon light-to-pulse circuit 300 by incorporating shieldedlight transducer 144 and associated electronics. Shielded lighttransducer 144 preferably has the same construction as exposed lighttransducer 106. However, shielded light transducer 144 does not receivelight 104. Charge generated by shielded light transducer 144, therefore,is only a function of noise. This noise is predominantly thermal innature. If shielded light transducer 144 has the same construction asexposed light transducer 106, the noise signal produced by shieldedlight transducer 144 will closely approximate the same noise within thesignal produced by exposed light transducer 106. By subtracting thesignal produced by shielded light transducer 144 from the signalproduced by exposed light transducer 106, the effect of noise in lighttransducer 106 can be greatly reduced.

Reset signal 308 controls switch 382 connected between shieldedtransducer output 384 and V_(DD). Sample signal 314 controls switch 386connected between shielded transducer output 384 and noise storagecapacitor 388, indicated by C_(SN). The voltage across noise storagecapacitor 388, noise storage capacitor voltage 390, is one input tocomparator 392. The second input to comparator 392 is ramp voltage 322.The output of comparator 392, noise comparator output 394, andcomparator output 332 serve as inputs to exclusive-OR gate 396.Exclusive-OR gate 396 generates exclusive-OR output 398 indicating theintensity of light 104.

Referring now to FIG. 16, a timing diagram illustrating operation of thelight sensor of FIG. 15 is shown. Light-to-pulse circuit 380 functionsin the same manner as light-to-pulse circuit 300 with regard to resetsignal 308, sample signal 314, light storage capacitor voltage 318, rampvoltage 322, ramp control signal 328, and comparator output 332. At time340, sample signal 314 is asserted while reset signal 308 is asserted.Switches 382 and 386 are both closed charging noise storage capacitor388 to V_(DD) as indicated by voltage level 410 in noise storagecapacitor voltage 390. At time 344, reset signal 308 is deassertedopening switch 382 and causing declining voltage 412 in noise storagecapacitor voltage 390 from charge produced by shielded light transducer144 due to noise. At time 354, sample signal 314 is deasserted endingintegration period 346 for noise collection. At time 358, ramp controlsignal 328 is deasserted causing declining voltage 360 in ramp voltage322. Initially, as indicated by voltage level 414, noise comparatoroutput 394 is unasserted because ramp voltage 322 is greater than noisestorage capacitor voltage 390. Since comparator output 332 is alsounasserted, output 398 from comparator 396 is unasserted as indicated byvoltage level 416. At time 418, ramp voltage 322 drops below the levelof noise storage capacitor voltage 390, causing noise comparator output394 to become asserted. Since noise comparator output 394 and comparatoroutput 332 are different, output 398 from comparator 396 is asserted. Attime 364, ramp voltage 322 drops beneath the level of light storagecapacitor voltage 318, causing comparator output 332 to become asserted.Since both noise comparator output 394 and comparator output 332 are nowasserted, output 398 from exclusive-OR gate 396 now becomes unasserted.The difference between time 364 and time 418, output pulse duration 420,has a time period proportional to the intensity of light 104 incident onexposed light transducer 106 less noise produced by shielded lighttransducer 144 over integration period 346. The duration between time418 and time 358, noise duration 422, is directly proportional to theamount of noise developed by shielded light transducer 144 overintegration period 346. Since the majority of this noise is thermalnoise, noise duration 422 is indicative of shielded light transducer 144temperature. At time 366, ramp control signal 328 is asserted,deasserting both noise comparator output 394 and comparator output 332.

Referring now to FIG. 17, a schematic diagram of an implementation ofthe light sensor of FIG. 15 using photodiodes as light transducers isshown. Light-to-pulse circuit 380 is implemented using exposedphotodiode 430 for exposed light transducer 106 and shielded photodiode432 for shielded light transducer 144. The anode of exposed photodiode430 is connected to ground and the cathode connected through transistorQ20 to V_(DD). The base of transistor Q20 is controlled by reset signal308. Hence, transistor Q20 functions as switch 310. Transistors Q21 andQ22 are connected in series between V_(DD) and ground to form a buffer,shown generally by 434. The base of transistor Q21 is connected to thecollector of exposed photodiode 430. The base of load transistor Q22 isconnected to fixed voltage V_(B). The output of buffer 434 is connectedthrough transistor Q23 to light storage capacitor 304. The base oftransistor Q23 is driven by sample signal 314, permitting transistor Q23to function as switch 316. The anode of shielded photodiode 432 isconnected to ground and the cathode is connected to V_(DD) throughtransistor Q24. The base of transistor Q24 is driven by reset signal 308permitting transistor Q24 to function as switch 382. Transistors Q25 andQ26 form a buffer, shown generally by 436, isolating the output fromshielded photodiode 432 in the same manner that buffer 434 isolatesexposed photodiode 430. Transistor Q27 connects the output of buffer 436to noise storage capacitor 388. The base of transistor Q27 is driven bysample signal 314 permitting transistor Q27 to function as switch 386.Typically, light storage capacitor 304 and noise storage capacitor 388are 2 pF. Ramp capacitor 324, typically 10 pF, is charged to V_(DD)through transistor Q28. The base of transistor Q28 is driven by rampcontrol signal 328 permitting transistor Q28 to function as switch 330.Ramp capacitor 324 is discharged through current source 326 at anapproximately constant current I_(R) of 0.1 μA when transistor Q28 isoff.

Sensor power-up response is improved and the effective dynamic rangeextended by including circuitry to inhibit output if ramp voltage 322drops beneath a preset voltage. Light-to-pulse circuit 380 includescomparator 438 comparing ramp voltage 322 with initialization voltage(V_(INIT)) 440. Comparator output 442 is ANDed with exclusive-OR output396 by AND gate 444 to produce AND gate output 446. During operation, iframp voltage 322 is less than initialization voltage 440, output 446 isdeasserted. The use of comparator 438 and AND gate 444 guarantee thatoutput 446 is not asserted regardless of the state of light-to-pulsecircuit 380 following power-up. In a preferred embodiment, theinitialization voltage is 0.45 V.

Sensor logic 306 generates control signals 308, 314, 328 based onintegration pulse 140 which may be generated internally or provided froman external source. Buffer 447 receives integration pulse 140 andproduces sample control 314. An odd number of sequentially connectedinverters, shown generally as inverter train 448, accepts sample control314 and produces reset control 308. A second set of odd numberedsequentially connected inverters, shown generally as inverter train 449,accepts reset signal 308 and produces ramp control signal 328. Thecircuit shown in FIG. 17 has a resolution of at least 8 bits and asensitivity of approximately 1 volt per lux-second. The maximum outputpulse duration 420 is independent of integration period 346 provided bythe duration of integration pulse 140.

Referring now to FIGS. 18-21, various embodiments for light sensorpackaging, output, and control are shown. Each embodiment may includelight-to-pulse circuitry as described with regard to FIGS. 13-17 above.In FIG. 18, light sensor package 450 accepts four pins for supplyvoltage V_(DD), ground, integration period signal 452, and output signal454. Integration period signal 452 may be integration pulse 140 used bylight-to-pulse circuit 380 to produce output 398 which is sent as outputsignal 454. In FIG. 19, light sensor package 456 requires only threepins for V_(DD), ground, and combined integration period and outputsignal 458. Combined signal 458 may be interconnect signal 114 asdescribed with regard to FIGS. 4 and 5 above. In FIG. 20, light sensorpackage 460 admits three pins for output signal 454, ground, andcombined V_(DD) and integration period signal 462. As is known in theart, combined signal 462 may be separated into power supply voltageV_(DD) and integration period signal 452 through the use of filters. InFIG. 21, light sensor package 464 admits three pins for V_(DD), ground,and output signal 454. Integration period signal 452 is generated withinlight sensor package 464 as described with regard to FIG. 22 below.

Referring now to FIG. 22, a block diagram of sensor logic for internallydetermining the integration period signal is shown. Sensor logic 306 mayinclude free-running counter 470 driven by internal oscillator 472.Counter 470 may have taps, one of which is indicated by 474, connectedto different counter bits. For example, one tap 474 may be connected tothe n^(th) bit, the next tap 474 to the n^(th)+2 bit, the next tap 474connected to the n^(th)+4 bit, and so on, with each successive tapthereby providing a pulse with a period four times longer than thepreceding tap 474. Sensor control signal generator 476 controls switch478 to determine which tap 474 will be used to produce integration pulse140. Typically, sensor control signal generator 476 sequences througheach tap 474 repeatedly. Sensor control signal generator 476 then usesintegration pulse 140 to generate control signals such as reset signal308, sample signal 314, and ramp control signal 328 as described withregards to FIG. 17 above.

Referring now to FIG. 23, a block diagram illustrating the use of lighttransducers having different effective areas to achieve increaseddynamic range is shown. As an alternative to or together with varyingthe integration time, pairs of exposed light transducer 106 and shieldedlight transducer 144 having different effective areas may be used. Ifphotodiodes 430,432 are used as light transducers 106,144, the effectivearea is the photodiode collector area. Small exposed light transducer490 produces charge which is converted to a voltage by light-to-voltagecircuit 492. Light-to-voltage circuit 492 may be implemented usingswitches 310, 316, and light storage capacitor 304 as described withregard to FIG. 15 above. Charge produced by small shielded lighttransducer 494 is converted to voltage by noise-to-voltage circuit 496.Noise-to-voltage circuit 496 may be implemented using switches 382, 386and noise storage capacitor 388 as described with regard to FIG. 15above. The outputs of light-to-voltage circuit 492 and noise-to-voltagecircuit 496 are converted to a pulse with a width based on chargeaccumulated by small exposed light transducer 490 less charge due tonoise integrated by small shielded light transducer 494 over anintegration period by voltage-to-pulse circuit 498. Voltage-to-pulsecircuit 498 may be implemented using comparators 320, 392, capacitor324, current source 326, and gate 396 as described with regard to FIG.15 above. Medium exposed light transducer 500 has an effective arealarger than the effective area for small exposed light transducer 490,resulting in increased sensitivity. For example, if the effective areaof medium exposed light transducer 500 is four times larger than theeffective area of small exposed light transducer 490, medium exposedlight transducer 500 will be four times more sensitive to light 104 thanwill be small exposed light transducer 490. Medium shielded lighttransducer 502 has an effective area the same as medium exposed lighttransducer 500. Additional light-to-voltage circuit 492,noise-to-voltage circuit 496, and voltage-to-pulse circuit 498 produce anoise-corrected output pulse with width based on light 104 incident onmedium exposed light transducer 500. Similarly, large exposed lighttransducer 504 and large shielded light transducer 506 provide stillincreased sensitivity over medium exposed light transducer 500 andmedium shielded light transducer 502 by having still greater effectivearea.

Switch 508 under the control of sensor logic 306 sets which output fromvoltage-to-pulse circuits 498 will be used for output signal 454. Outputsignal 454 may be selected based on a signal generated within sensorlogic 306 or may be based on a signal provided from outside of sensorlogic 306.

In an alternative embodiment, only one shielded fight transducer 144 isused. The output of shielded light transducer 144 is scaled prior toeach noise-to-voltage circuit 496 in proportion to the varying effectiveareas of exposed light transducers 106. It will be recognized by one ofordinary skill in the art that, though the examples shown in FIG. 23 hasthree pairs of exposed light transducer 106 and shielded lighttransducer 144, any number of pairs may be used.

Referring now to FIG. 24, a block diagram illustrating the use of lighttransducers having different apertures to achieve increased dynamicrange is shown. As an alternative to or together with specifying theintegration period, exposed light transducers 106 having the sameeffective area may each have a different aperture admitting area foradmitting light 104. Varying apertures may be produced using partialshield 520 blocking light 104 from reaching a portion of exposed lighttransducer 106. Each exposed light transducer 106 produces chargeconverted to a voltage by a corresponding light-to-voltage circuit 492.Switch 522 under the control of sensor logic 306 selects which output oflight-to-voltage circuits 492 to forward to voltage-to-pulse circuit498. Voltage-to-pulse circuit 498 produces output signal 454 compensatedfor noise sensed by shielded light transducer 144 and processed bynoise-to-voltage circuit 496. Sensor logic 306 may select output oflight-to-voltage circuits 492 based on an internally generated controlsignal or on a control signal received from outside of sensor logic 306.

Referring now to FIG. 25, a schematic diagram illustrating differenttransducer capacitances for different amounts of light-induced charge toachieve increased dynamic range is shown. A photodiode, shown generallyby 530, is formed by n-type diffusion 532 in p-type substrate 534. Light104 incident on photodiode 530 generates charge 536 which may beaccumulated in photodiode well 538 beneath n-type diffusion 532.Photodiode 530 has intrinsic photodiode capacitance C_(PD). Floatingdiffusion 540 is also formed by diffusing n-type material in substrate534. Floating diffusion 540 is connected through transistor Q20 to resetvoltage V_(RESET). The gate of transistor Q20 is connected to resetsignal 308 under the control of sensor logic 306. Floating diffusion 540is also connected to the input of buffer 542. The output of buffer 542is transducer output V_(OUT). Floating diffusion 540 defines diffusionwell 544 formed in a region of substrate 534 when reset signal 308 isasserted. Floating diffusion 540 has an intrinsic floating diffusioncapacitance C_(FD). Transmission gate 546 is positioned betweendiffusion 532 and floating diffusion 540. Transmission gate 546 is heldat voltage V_(TG) to form transmission well 548 thereunder. Transmissionwell 548 has a depth shallower than photodiode well 538 and diffusionwell 544. Transmission gate 546 has an intrinsic transmission gatecapacitance C_(TG).

When reset signal 308 is asserted, bringing floating diffusion 540 toV_(RESET), charge is eliminated in diffusion well 544. Further, whencharge is reset in diffusion well 544, any charge 536 in photodiode well538 above the depth of transmission well 548 flows through transmissionwell 548, through floating diffusion 540, and is eliminated. During alight integration period, reset signal 308 is unasserted, causing thevoltage of floating diffusion 540 to float based on the amount of charge536 in diffusion well 544. As light 104 strikes diffusion 532, charge536 is created. Since charge 536 in photodiode well 538 up to the levelof transmission well 548 was not eliminated by charge reset, additionalcharge 536 produced by incident light 104 flows from photodiode well 538through transmission well 548 and into diffusion well 544. At chargelevel 550, beneath the level of transmission well 548, only diffusionwell 544 is filling with charge 536. Hence, the voltage of floatingdiffusion 540 is inversely proportional to floating gate capacitanceC_(FD). When enough charge 536 has been generated to fill diffusion well544 above the level of transmission well 548 such as, for example level552, diffusion well 544, transmission well 548, and photodiode well 538all fill with charge 536. Hence, the voltage of floating diffusion 540is now inversely proportional to the sum of floating diffusioncapacitance C_(FD), transmission gate capacitance C_(TG), and photodiodecapacitance C_(PD).

Referring now to FIG. 26, a graph of output potential as a function ofaccumulated incident light for the transducer of FIG. 25 is shown. Acurve, shown generally by 554, shows transducer output V_(OUT) as afunction of light 104 incident on diffusion 532 and, possibly, floatingdiffusion 540 over the integration period. During steep portion 556,charge 536 is accumulating in diffusion well 544 alone. Since theconversion gain is based only on floating diffusion capacitance C_(FD),photodiode 530 appears to have a high sensitivity to incident light 104.During shallow portion 558, charge 536 is accumulated in diffusion well544, transmission well 548, and photodiode well 538. Since theconversion gain is now dependent on the parallel combination ofcapacitances C_(FD), C_(TG), and C_(PD), photodiode 530 now appears lesssensitive to incident light 104. By adjusting voltages V_(RESET) andV_(TG), knee point 559 between steep portion 556 and shallow portion 558can be shifted affecting the dynamic range. For example, if the maximumvoltage swing for floating diffusion 540 is 1 volt; the ratio of C_(FD)to the sum of C_(FD), C_(TG), and C_(PD) is 1:100; and knee point 559 isset at 0.5 volts, the dynamic range of photodiode 530 is increased about50 times over the dynamic range of a similar photodiode without dualcapacitance.

Referring now to FIG. 27, a schematic diagram illustrating a photodiodetransducer incorporating an anti-bloom gate according to an embodimentof the present invention is shown. Anti-bloom gate 560 is formed betweendiffusion 532 and source voltage diffusion 562 tied to V_(DD).Anti-bloom gate 560 is tied to anti-bloom voltage V_(AB). Anti-bloomgate 560 forms anti-bloom well 564 in substrate 534 between photodiodewell 538 and source diffusion well 566. Anti-bloom voltage V_(AB) isless that transmission gate voltage V_(TG) well 564, making anti-bloomwell 564 shallower than transmission well 548. When accumulated chargegenerated by photodiode 530 exceeds charge level 568 equal to the depthof anti-bloom well 564, the excess charge flows beneath anti-bloom gate560 into source voltage diffusion 562 and is eliminated. Anti-bloom gate560 prevents output voltage V_(OUT) from dropping below a leveldetectable by comparator 320 in light-to-pulse circuit 380.

Referring now to FIG. 28, a drawing illustrating enclosure for a lightsensor according to an embodiment of the present invention is shown.Light sensor 48,52 includes enclosure 100 having window 102 foradmitting light, one ray of which is indicated by 570. Enclosure 100admits power pin 108, ground pin 110, and signal pin 112. Semiconductordie 572, encapsulated within enclosure 100, incorporates lighttransducers 106,144 and associated electronics as described with regardsto FIGS. 4-5 and 13-26 above. Pins 108,110,112 may be wire bonded to die527, as shown by wire 574 for power pin 108 and wire 576 for signal pin112, or may be directly bonded to die 527, as shown for ground pin 110.

Preferably, enclosure 100 is the same type used to constructthree-terminal light emitting diodes (LEDs). A preferred format iscommonly referred to as the T-1¾ or 5 mm package. Encapsulatingelectronics in such packages is well known in the art of opticalelectronics manufacturing.

A lens, shown generally by 578, is preferably used to focus light ontoexposed light transducer 106. Lens 578 may be placed in front of lightsensor 48,52 or, preferably, may be incorporated into window 102 asshown in FIG. 28. Lens 578 defines the field of view of light sensor48,52 and provides improved sensitivity through optical gain.

Referring now to FIG. 29, a graph illustrating light sensor field ofview as a function of light transducer distance from the lens is shown.The field of view for exposed light transducer 106 in light sensor 48,52is defined as view angle θ made by marginal ray 570 with respect tooptical axis 580 through exposed light transducer 106. The half anglefield of view for spherical lens 578 is expressed by Equation 3:$\begin{matrix}\begin{matrix}{\theta = \quad {90 - {\arccos ( \frac{r}{R} )} +}} \\{\quad {{\frac{n_{2}}{n_{1}} \cdot \sin}\quad ( {{\arccos \quad ( \frac{r}{R} )} - {\arctan ( \frac{d - ( {R - \sqrt{R^{2} - r^{2}}} )}{r} )}} )}}\end{matrix} & (3)\end{matrix}$

where r is the lens aperture radius, R is the radius of curvature oflens 578, n₂ is the index of refraction of material within enclosure100, n₁, is the index of refraction outside of enclosure 100, d is thedistance from the center of lens 578 to exposed light transducer 106,and θ is measured in degrees. Typically, T-1¾ enclosure 100 is filledwith epoxy and sensor 48,52 operates in air making the ratio of n₂ ton₁, approximately 1.5. Curve 590 plots half angle field of view θ as afunction of distance d for a T-1¾ enclosure having a spherical lens 578with radius R of 5.0 mm. As light transducer 106 moves farther from lens578, the field of view decreases.

Referring now to FIG. 30, a graph illustrating light sensor optical gainas a function of light transducer distance from the lens is shown.Assuming paraxial approximation for rays 570, the optical gain of lens578 can be estimated by considering the ratio of additional opticalenergy collected by light transducer 106 with lens 578 to the opticalenergy collected by light transducer 106 without lens 578. This can becomputed by considering a cone of light with a base at the surface oflens 578 and a point at the focal point of lens 578. The optical gain Gmay then be expressed as a function of the ratio of the cross section ofthe cone to the area of light transducer 106 which reduces to Equation4: $\begin{matrix}{G = \frac{f^{2}}{( {f - d} )^{2}}} & (4)\end{matrix}$

Curve 600 shows optical gain G as a function of distance d for a T-1¾enclosure having a spherical lens 578 with radius R of 5.0 mm and afocal length of 15.0 mm. As light transducer 106 moves farther from lens578, the optical gain increases.

The distance d between lens 578 and light transducer 106 can be adjustedfor optimal performance of ambient light sensor 48 and glare sensor 52.Ambient light sensor 48 should have a wide field of view but need not beas sensitive as glare sensor 52. Glare sensor 52 should have a narrowerfield of view but must be more sensitive and, therefore, benefits from ahigher optical gain. For the lens described with regards to FIGS. 29 and30 above, a distance d of between 2 mm and 3 mm is suitable for ambientlight sensor 48 and a distance d of between 6 mm and 7 mm is suitablefor glare sensor 52. In addition to modifying lens parameters, otherlens types such as aspheric, cylindrical, and the like are possiblewithin the spirit and scope of the present invention.

Referring now to FIG. 31, a graph illustrating frequency response of thehuman eye is shown. Curve 610 is the relative photopic or daylightfrequency response of the human eye. Curve 612 is the relative scotopicor night frequency response of the human eye. In addition to being moresensitive to light intensity, scotopic response 612 is shifted moretowards violet than photopic response 610. In order to preserve nightvision, which degrades rapidly when exposed to bright light particularlyin the range of scotopic curve 612, exposed light transducer 106 shouldhave a frequency response similar to scotopic curve 612. If this is notpractical, exposed light transducer 106 should at least have anattenuated infrared response. This is increasingly more important ashigh intensity discharge (HID) headlamps, which emit more blueish lightthan do incandescent or halogen lamps, gain in popularity.

Referring now to FIG. 32, a graph illustrating frequency response of atypical light transducer is shown. The relative frequency response of atypical photodiode is shown as curve 620. When compared to scotopicresponse curve 612, the frequency response of exposed light transducer106 contains significantly more infrared sensitivity. As described withregards to FIG. 2 above, filter 58,60 my be placed before orincorporated into sensor 48,52 so that the output of exposed lighttransducer 106 more closely resembles scotopic frequency response 612 ofthe human eye.

Referring now to FIG. 33, a drawing of an enclosure incorporating aninfrared filter according to an embodiment of the present invention isshown. Window 102 in enclosure 100 includes infrared filter 630operative to attenuate infrared components of light rays 570 strikingexposed light transducer 106. Infrared filter 630 may be a hot mirroravailable from Optical Coating Laboratories, Inc. of Santa Rosa, Calif.A lens, such as described with regards to FIGS. 28-30 above, may beplaced in front of infrared filter 630. Additional filtering for exposedlight transducer 106 is described in U.S. Pat. No. 4,799,768 titled“Automatic Rearview Mirror with Filtered Light Sensors” to Gahan, whichis incorporated herein by reference.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, it is intended that thefollowing claims cover all modifications and alternative designs, andall equivalents, that fall within the spirit and scope of thisinvention.

What is claimed is:
 1. A rearview mirror for an automotive vehicleallowing a vehicle operator to view a scene generally behind the vehicleoperator, the mirror comprising: a dimming element having a variablyreflective surface, the degree of reflectivity based on a controlsignal; an ambient light sensor positioned to receive light from aregion generally in front of the vehicle, the ambient light sensoroperative to output a discrete ambient light signal based on the amountof light incident on the ambient light sensor over an ambient lightintegration period; a glare sensor positioned to view the scenegenerally behind the vehicle operator, the glare sensor operative tooutput a discrete glare signal based on the amount of light incident onthe glare sensor over a glare integration period; and a dimming logic incommunication with the dimming element, the ambient light sensor, andthe glare sensor, the dimming logic operative to (a) determine anambient light level based on the ambient light signal, (b) determine theglare integration period based on the determined ambient light level,(c) determine a mirror glare level based on the glare signal, and (d)determine the dimming element control signal based on the determinedmirror glare level.
 2. A rearview mirror as in claim 1 wherein theambient light sensor has an input for receiving an ambient lightintegration period signal specifying the ambient light integrationperiod, the dimming logic further operative to determine the ambientlight integration period based on at least one previously determinedambient light level and to output the ambient light integration periodsignal based on the determined ambient light integration period.
 3. Arearview mirror as in claim 1 wherein the ambient light sensor has aninput for receiving an ambient light integration period signalspecifying the ambient light integration period and wherein the ambientlight signal is a pulse having a pulse width indicative of the ambientlight level, the dimming logic further operative to: generate a sequenceof integration period signals, each integration period signal in thesequence specifying a different light integration period; and determinethe ambient light level based on a resulting ambient light signal havinga pulse width within at least one preset width threshold.
 4. A rearviewmirror as in claim 1 wherein at least one of the ambient light sensorand the glare sensor is a light sensor comprising: a light transducerexposed to light operative to accumulate charge in proportion to lightincident over a light integration period; and a sensor logic incommunication with the exposed light transducer, the sensor logicoperative to (a) determine the light integration period prior tobeginning integration, (b) reset the charge accumulated in the exposedlight transducer at the beginning of the determined light integrationperiod, (c) measure the charge accumulated by the exposed lighttransducer over the determined light integration period, and (d) outputa pulse having a width based on the measured accumulated exposed lighttransducer charge.
 5. A rearview mirror as in claim 4 wherein the lightsensor further comprises a light transducer shielded from ambient light,the shielded light transducer having the same construction as theexposed light transducer, the shielded light transducer operative toaccumulate charge in proportion to noise over the integration period,wherein the sensor logic is further operative to: reset the chargeaccumulated in the shielded light transducer at the beginning of thedetermined light integration period; measure the charge accumulated bythe shielded light transducer over the determined light integrationperiod; and output a pulse having a width based on the differencebetween the measured accumulated exposed light transducer charge and themeasured accumulated shielded light transducer charge.
 6. A rearviewmirror as in claim 5 wherein the light sensor is further operative to:receive an integration pulse, the width of the integration pulsedetermining the integration period; and generate the output pulse afterthe integration pulse, the difference in time between the end of theintegration pulse and the start of the output pulse indicative of theamount of thermal noise in the light sensor.
 7. A rearview mirror as inclaim 6 wherein the dimming logic is further operative to: determine theamount of time between the end of the integration pulse and the start ofthe output pulse; and determine the light sensor temperature based onthe determined amount of time.
 8. A rearview mirror as in claim 7wherein the dimming logic is further operative to disable automaticdimming of the dimming element reflective surface if the light sensortemperature exceeds a preset limit.
 9. A rearview mirror as in claim 1wherein at least one of the ambient light sensor and the glare sensor isa light sensor comprising: an enclosure having a window for receivinglight, the enclosure admitting a power pin, a ground pin, and a signalpin; an exposed light transducer disposed within the enclosure, theexposed light transducer operative to accumulate charge in proportion tolight received through the window incident on the exposed lighttransducer; a light-to-pulse circuit disposed within the enclosure andin communication with the exposed light transducer, the power pin, andthe ground pin, the light-to-pulse circuit operative to output an outputpulse, the output pulse width based on charge accumulated by the exposedlight transducer over an integration period; and a sensor logic disposedwithin the enclosure, the sensor logic in communication with thelight-to-pulse circuit, the power pin, the ground pin, and the signalpin, the sensor logic operative to (a) receive an integration pulse onthe signal pin, (b) determine the integration period based on the widthof the integration pulse, and (c) output the output pulse on the signalpin.
 10. A rearview mirror as in claim 9 wherein the dimming logiccomprises at least one signal pin, each dimming logic signal pinconnected to the signal pin of the light sensor, for each signal pin thedimming logic further operative to: set the dimming logic signal pin tooutput mode; determine an integration period; generate an integrationpulse on the dimming logic signal pin, the width of the integrationpulse based on the determined integration period; set the dimming logicsignal pin to input mode; receive the light sensor output pulse; anddetermine a light level received by the light sensor based on the lightsensor output pulse.
 11. A rearview mirror as in claim 10 wherein thedimming logic determines an integration period by cycling through asequence of predetermined integration periods.
 12. A rearview mirror asin claim 1 wherein the dimming element is an electrochromic element. 13.A rearview mirror as in claim 12 wherein the dimming logic generates thedimming element control signal as a constant voltage between transitionpoints, the time between adjacent transition points being a fixedtransition period, wherein, at each transition point, the dimming logicoutputs a high voltage if an actual electrochromic element input voltageis less than a desired electrochromic element input voltage and outputsa low voltage otherwise.
 14. A rearview mirror as in claim 13 whereinthe dimming logic comprises a low pass filter operative to filter thedimming element control signal to produce the actual electrochromicelement input voltage.
 15. A rearview mirror as in claim 13 wherein thedesired electrochromic element input voltage is a digital number, thedimming logic comprising an analog-to-digital converter operative todigitize the actual electrochromic element input voltage.
 16. A rearviewmirror as in claim 13 wherein the desired electrochromic element inputvoltage is a digital number, the dimming logic comprising: adigital-to-analog converter operative to produce a desired voltage levelrepresenting the desired electrochromic element input voltage number;and a comparator accepting the desired voltage level and the actualelectrochromic element input voltage, the comparator operative toindicate if the desired voltage level is greater than the actualelectrochromic element input voltage.
 17. A rearview mirror as in claim13 wherein the transition period is set to inhibit flicker in theelectrochromic element that may be noticed by the vehicle operator. 18.A rearview mirror as in claim 13 wherein the transition period isbetween two seconds and two microseconds.
 19. A rearview mirror as inclaim 13 further comprising a switch connected across the electrochromicelement, the dimming logic further operative to close the switch whenthe actual electrochromic element input voltage exceeds the desiredelectrochromic element input voltage by more than a preset amount.
 20. Arearview mirror as in claim 1 wherein the dimming logic is furtheroperative to output a signal based on the determined ambient lightlevel.
 21. A rearview mirror as in claim 1 further comprising a glarelens for the glare sensor, the glare lens providing the glare sensorwith a narrower field of view than the ambient light sensor field ofview, the glare lens further providing the glare sensor with a higheroptical gain than the ambient light sensor optical gain.
 22. A rearviewmirror as in claim 1 wherein the dimming logic determines the glareintegration period based on the binary logarithm of the ambient lightlevel, the dimming logic further operative to: obtain the ambient lightlevel as a binary number; determine a first binary number portion basedon the bit position of the most significant binary one in the ambientlight level binary number; determine a second binary number portionbased on the bit pattern following the most significant binary one inthe ambient light level binary number; and determine as the ambientlight level binary logarithm as the concatenation of the first binarynumber portion and the second binary number portion.
 23. A dimming logicoperative to generate a control signal for controlling variabletransmissivity of an electrochromic element based on a desired controlvoltage, the electrochromic element dimming in response to an appliedcontrol voltage, the dimming logic operative to generate the controlsignal as a constant voltage between transition points, the time betweenadjacent transition points being a fixed transition period wherein, ateach transition point, the dimming logic outputs a high voltage if theapplied control voltage is less than the desired control voltage andoutputs a low voltage otherwise.
 24. A dimming logic as in claim 23wherein the dimming logic comprises a low pass filter operative tofilter the control signal to produce the applied control voltage.
 25. Adimming logic as in claim 23 wherein the desired control voltage is adigital number, the dimming logic comprising an analog-to-digitalconverter operative to digitize the applied control voltage.
 26. Adimming logic as in claim 23 wherein the desired electrochromic elementinput voltage is a digital number, the dimming logic comprising: adigital-to-analog converter operative to produce a desired voltage levelrepresenting the desired control voltage number; and a comparatoraccepting the desired voltage level and the applied control voltage, thecomparator operative to indicate if the desired voltage level is greaterthan the applied control voltage.
 27. A dimming logic as in claim 23wherein the transition period is set to inhibit flicker in theelectrochromic element.
 28. A dimming logic as in claim 23 wherein thetransition period is between two seconds and two microseconds.
 29. Adimming logic as in claim 23 further comprising a switch connectedacross the electrochromic element, the dimming logic further operativeto close the switch when the applied control voltage exceeds the desiredcontrol voltage by more than a preset amount.
 30. A dimming logic as inclaim 29 wherein the comparison logic comprises an analog-to-digitalconverter operative to digitize the applied control voltage.
 31. Adimming logic as in claim 29 wherein the comparison logic comprises: adigital-to-analog converter operative to produce a desired voltage levelrepresenting the desired control level; and a comparator accepting thedesired voltage level and the applied control voltage, the comparatoroperative to indicate if the applied control voltage is greater than thedesired voltage level.